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ChemistryClass 113 marksmedium
Some Basic Concepts of Chemistry
How many molecules and how many atoms are present in 0.25 mol of CO2? (Avogadro number = 6.022 x 10^23)
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Number of molecules = 0.25 x 6.022 x 10^23 = 1.506 x 10^23 molecules. Each CO2 molecule has 3 atoms (1 C + 2 O), so number of atoms = 3 x 1.506 x 10^23 = 4.52 x 10^23 atoms.
N = n x N_A
Marking-scheme points
- ✓molecules = n x N_A = 0.25 x 6.022e23 = 1.506e23
- ✓Atoms per CO2 molecule = 3
- ✓atoms = 3 x 1.506e23 = 4.52e23
ChemistryClass 113 markshard
Some Basic Concepts of Chemistry
A compound contains 24.27% carbon, 4.07% hydrogen and 71.65% chlorine by mass. Its molar mass is 98.96 g/mol. Find its empirical and molecular formula.
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Divide each percentage by atomic mass: C = 24.27/12 = 2.02, H = 4.07/1 = 4.07, Cl = 71.65/35.5 = 2.02. Divide by the smallest (2.02): C = 1, H = 2, Cl = 1. Empirical formula = CH2Cl (empirical mass = 12 + 2 + 35.5 = 49.5). n = molar mass / empirical mass = 98.96/49.5 = 2. Molecular formula = C2H4Cl2.
n = molar mass / empirical formula mass
Marking-scheme points
- ✓Moles of atoms: C 2.02, H 4.07, Cl 2.02
- ✓Simplest ratio 1 : 2 : 1 -> empirical CH2Cl (mass 49.5)
- ✓n = 98.96/49.5 = 2 -> molecular C2H4Cl2
ChemistryClass 113 marksmedium
Some Basic Concepts of Chemistry
Calculate the molarity of a solution prepared by dissolving 5 g of NaOH in enough water to make 450 mL of solution.
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Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol. Moles of NaOH = 5/40 = 0.125 mol. Volume = 450 mL = 0.450 L. Molarity = 0.125 / 0.450 = 0.278 M (approximately 0.28 M).
M = n / V(L)
Marking-scheme points
- ✓Molar mass NaOH = 40 g/mol
- ✓moles = 5/40 = 0.125 mol
- ✓M = 0.125/0.450 = 0.28 M
ChemistryClass 113 markshard
Some Basic Concepts of Chemistry
3.0 g of H2 reacts with 29 g of O2 to form water (2H2 + O2 -> 2H2O). Identify the limiting reagent and calculate the mass of water formed.
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Structure of Atom
State the main postulates of Bohr's model of the hydrogen atom.
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Structure of Atom
The radius of the first Bohr orbit of hydrogen is 0.529 Angstrom. Calculate the radius of the third orbit.
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Structure of Atom
Calculate the de Broglie wavelength of an electron moving with a velocity of 2.05 x 10^7 m/s. (mass of electron = 9.1 x 10^-31 kg, h = 6.626 x 10^-34 J s)
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Structure of Atom
What are the four quantum numbers? State what each one describes.
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Structure of Atom
Write the electronic configurations of chromium (Z = 24) and copper (Z = 29). Explain why they are exceptions to the expected order.
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Classification of Elements and Periodicity
How does atomic radius vary across a period and down a group? Give reasons.
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Classification of Elements and Periodicity
Define ionization enthalpy. Explain its trend across a period and down a group.
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Classification of Elements and Periodicity
What are s-, p-, d- and f-block elements? Why are d-block elements called transition elements?
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Chemical Bonding and Molecular Structure
Using VSEPR theory, predict the shapes and bond angles of BF3, NH3 and H2O.
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Chemical Bonding and Molecular Structure
Describe the hybridization, shape and bonding in an ethyne (C2H2) molecule.
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Chemical Bonding and Molecular Structure
Using molecular orbital theory, write the molecular orbital configuration of O2, calculate its bond order and explain its magnetic nature.
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Chemical Bonding and Molecular Structure
State Fajans' rules governing the covalent character of an ionic bond.
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Chemical Bonding and Molecular Structure
What is dipole moment? Why is the dipole moment of CO2 zero while that of H2O is not?
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States of Matter
A gas occupies 300 mL at 27 deg C. What volume will it occupy at 127 deg C if the pressure is kept constant?
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States of Matter
Calculate the volume occupied by 2 moles of an ideal gas at 300 K and a pressure of 2 atm. (R = 0.0821 L atm K^-1 mol^-1)
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States of Matter
State the main postulates of the kinetic molecular theory of gases.
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